


i gave an inch (you took a mile)

by sweetsinnerchild



Category: Undertale (Video Game)
Genre: Ectogenitals, M/M, Sexual Coercion, Underfell, Voyeurism
Language: English
Status: In-Progress
Published: 2016-03-05
Updated: 2016-10-09
Packaged: 2018-05-24 19:52:14
Rating: Mature
Warnings: Rape/Non-Con
Chapters: 5
Words: 12,462
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6164770
Author URL: https://archiveofourown.org/users/sweetsinnerchild/pseuds/sweetsinnerchild
Summary: <blockquote class="userstuff">
              <p>love in the underground is a lie.</p>
            </blockquote>





	1. harassment and molesting, tell your boss all about it

**Author's Note:**

  * Translation into 한국어 available: [빌미](https://archiveofourown.org/works/6351241) by [Osteophile](https://archiveofourown.org/users/Osteophile/pseuds/Osteophile)



> hoooo boy.
> 
> i have an intense need for noncon sanster and developing possessive papyrus and underfell answers all of my needs
> 
> this fic will be updated whenever i can find the time to stop kinkshaming myself

Sans knows that he’s weak.

He also knows weakness in the underground is a death sentence. In a world where LOVE and EXP are the main currency of survival, being weak means easy pickings for other monsters looking to climb the ranks in the hierarchy of fear or be feared. An ambush would immediately dust him and his one-hit HP.

The obvious solution is to get stronger, to gain LOVE by killing other monsters first. It wasn’t as if Sans couldn’t hold his own in a fight when he was ultimately forced to. If he saw them coming, he could at least dodge or deflect the initial blow, and then strike back; but no, he had figured out that his HP hadn’t budged when he had finally gained a LOVE, that he was physically weak and too easily tired to actually ever improve.

The alternative is to live under another monster’s protection, in exchange for loyalty and obedience. It isn’t a shameful thing - plenty of monsters do it, just to cling to the semblance of security and to put food on their plates. Those in power were always in need for those who would bow to their power after all.

It isn’t shameful, but it is necessary, especially when Sans has Papyrus.

Fighting becomes much harder when he has Papyrus to defend. Although Papyrus is healthier and stronger than he ever will be, he is still so young, and still so very optimistic. He had clung onto the idea that if there was good in his brother, there was good in everybody, and had hesitated when he needed to harm another monster.

(And Sans, fool that he was, had let him think that way. Maybe he himself wanted it to be true.)

(In the future, Papyrus, well. Papyrus doesn’t believe in that anymore.)

So here’s the other secret, why they were still brothers in a world that laughed at the concept of ‘family’: Sans knows that he’s _weak_.

He’s weak for affection, for acts of tenderness and words of praise, for that elusive of concept of love. He wants the warmth of a hug, the touch of a gentle hand, the acknowledgement that he’s done something right, something well. He wants all that in a world that measured worth by LOVE, not love, and he knows that wanting something that has never existed is a little more than a foolish dream.

But oh, how he wants.

And maybe that’s why he doesn’t abandon Papyrus and open his eyes to the ways of the world - because Papyrus would never hesitate to tug at his arm and call him ‘brother’, to hug him after another day survived, to cry when he had almost got dusted. Papyrus is what keeps him going, what keeps him alive even though somedays he imagines it would be easier if he was dead - but what would happen to Papyrus if he died?

So when Gaster saves them from an ambush that they wouldn’t have survived, Sans accepts his offer to be his assistant, in exchange for food and lodgings for both him and Papyrus. He needs help to keep Papyrus alive and happy, and he’d do anything and everything for that.

* * *

The premises of his job is easy: do whatever Gaster tells him.

On good days, this means preparing experiments, fetching whatever the other skeleton needs, and cleaning up after the experiments. Sans finds that despite his lack of knowledge, the theories Gaster tests are frankly fascinating. He makes excuses to pore over the books in the library, trying to understand what the Royal Scientist is attempting, and sometimes even daring to affirm his understanding with Gaster himself by appealing to his arrogant side. On good days, Gaster even obliges, and Sans can pretend that the gleam in his eyes is one of pride.

It’s the experiments Sans is less enthusiastic about - but he has come to find that he prefers handling monster dust over preserved human remains, if only because they are easier to clean.

After work he trudges back to his quarters where Papyrus waits, bringing with him food and interesting items he thinks Papyrus would like. He had lied to Gaster that Papyrus was too young to read and be of any help in the labs, and that Sans himself would teach him. The other skeleton had nodded impassively, his face betraying no particular emotion and Sans had been thankful that he hadn’t had other… interests in Papyrus, something other Boss monsters might have demanded.

On bad days, Sans learns what obedience entails.

(Gaster did have interests. Just not in Papyrus.)

* * *

It starts small. Small, lingering touches on his arms; a phalange brushing against the back of his neck, catching briefly on the vertebrae. Sans stills, but Gaster never says anything, only continues with his next experiments.

The touches get longer, firmer - fingers pressing along the length of his collarbones, curling into the dip of his shoulder blade, Gaster stepping up behind him, leaning over him to reach for something on the shelf, drawing away with a lingering touch on his metacarpals. Whenever something falls onto the floor, he can feel Gaster’s gaze on him as he bends down to retrieve the item, intrusive and unsettling.

_bear with it_ , Sans tells himself. There is nothing wrong with touching and looking. It’s better that these touches aren’t out to kill him. There’s nothing wrong.

So Sans keeps quiet when Gaster continues his ministrations, the touches only getting bolder. His palm drags down to the small of Sans’ back; his leg slides forward, resting against the back of Sans’ pelvis; his fingers curl around the jut of Sans’ hip as he crowds the smaller skeleton into a corner of the lab they worked in, experiment abandoned on the table.

“stop,” he finally snaps, his hand flying out to dislodge Gaster’s. Gaster removes his hand, but stays where he is. Though Sans is glaring right back at the older skeleton, he can feel how his back is pressing against the wall, see how unfazed Gaster’s gaze remains at his rejection.

“And here I’d thought you would wait until we were both naked,” the scientist says, his voice calm and measured and on this side of mocking. Sans bristles.

“i am not your personal whore,” he tells Gaster, because he didn’t sign up for this - and Gaster has the nerve to just laugh, an ugly bark that shows Sans exactly what he thinks about that.

“You’re supposed to do everything I tell you to,” Gaster repeats, his grin growing wide and wider. “Everything, including being ‘my personal whore’.“

He reaches for Sans again, and Sans reacts - he pings Gaster’s soul blue and throws him across the room, to get him as far away from him as he can, and runs towards the door of the lab. He has to get Papyrus out, they have to go - but just as Sans reaches the door, the cold grip of his boss’ magic on his soul slams him down onto the floor, just inches away from the door handle.

He hears Gaster standing up, brushing at his clothes, taking his own sweet time to saunter over to Sans’ prone body, held in place by magic without its master breaking a sweat. The footsteps stops, and the steady weight of a single foot presses down firmly onto the back of his ribs.

“That was the most stupid thing you could have done in your life,” Gaster says. Sans shivers at controlled anger beneath the smooth cadence of the other’s voice. “You’re usually so smart, Sans. You had potential, and that was why I saved you and your brother - but attacking me?” He tuts. "Perhaps I misjudged your intelligence.”

He struggles to get up, to throw Gaster’s magic off his soul - and Gaster grinds his foot sharply down, as if he was crushing an insect. Sans cries out, sharp and loud, and only then does Gaster lifts his foot.

“But perhaps I could allow this mistake,” Gaster says, his voice soft and deceptively gentle. “If this is to be the last time. What do you say, Sans?”

The grip around his soul eases ever so slightly, and Sans raises his head.

“if you’re going to kill me,” he snarls. “get on with it.”

“Kill you?” Gaster says. “Why would I do that?” There’s the sound of his lab coat dragging against the floor, and a boney hand brushes over the curve of his skull, as if he was being patted and if it weren’t so patronising Sans would have found it soothing. “You’re more useful alive to me.”

“i’m not letting you fuck me.“

The hand pauses, and then continues its repetitive motion. "What do you think would happen to your brother?”

The non sequitur was jarring, but the message is clear.

“don’t you fucking dare,“ and Sans begins to pull on the reserves of his magic, for the barest scraps in order to tear apart this bastard who would threaten Papyrus right here and right now, “you bastard, don’t you dare -”

The hand tightens around his skull in a crushing grip and slams it down onto the floor.

“Quiet,” Gaster is saying over the ringing pain in his head. Sans blinks profusely into the floor, his vision black and blurry, the dusty grey tiles of the laboratory swimming back into uncomfortable focus. “Let me tell you what will happen. You and your brother will be used as the test subjects in the next experiment. You will be the control, and watch while your brother receives the brunt of the testing. Then, later, I will, ah, fuck you, and use you however I like. Is that clear?”

Sans is going to kill him. Sans is going to gain a LOVE with this bastard’s death, and even Papyrus imploring him will not stop him.

“Or,” and the hand turns gentle, rubbing away at the pain, and Sans groans into the floor. “You let me take you, and I will continue keeping the both of you under my protection. I’ll even promise to not touch your brother. He can continue on living in blissful ignorance in your quarters, and you can pretend at his frankly nauseating ideas of world peace. But you and I know better than that, don’t we?

"And if you do manage to kill me,” Gaster laughs, low and dismissive. "Just think of how many monsters you have to fight off, and how many monsters would come after the both of you. Another challenger for the title of Royal Scientist. And another, and many, many more.”

The scientist stands up, and abruptly releases Sans’ soul from his hold. Sans stays on the floor, humiliated and defeated.

“I’ll be in my quarters,” Gaster says, turning his back to Sans. “Think about it, won’t you?“

He sweeps out of the room, and Sans does not move.


	2. open doors are meant for looking through

**Notes for the Chapter:**

> i think a lot of people expected something
> 
> and i think i either exceeded that expectation or fucked up everything
> 
> thanks for commenting i really want to reply but mostly it'll be screaming aaaaaaaa at all of you
> 
> brb kinkshaming myself
> 
> shoutout to beescream for bee-ing the best when i'm screaming at them about my insecurity

There’s something wrong with Sans. **  
**

There are many things wrong with Sans, of course, including his general sloppiness and his eating habits and how frail he actually is. Papyrus knows that Sans considers himself weak, but his brother isn’t weak. He’s kept them alive for so long, and even though the world tells them to kill or be killed, his brother has yet to give up on him. If that is any indication of the goodness in monsters, it is one Papyrus is willing to hold to.

But today Sans walks into the room like he expects an ambush. It’s a subtle thing, but years together means that Papyrus can tell when his brother is on alert, ready to run - and this is one of those times. There is a certain tension in the way his shoulders are hunched together, in the sharp edges of his plastered smile and in the way he looks at Papyrus, as if Papyrus was in danger.

Which is silly, because they aren’t in any danger, not after Gaster took them in.

And maybe Papyrus should be grateful that they’re safe now, with a roof over their heads and food on their plates, but Papyrus isn’t. Before, it had been him and Sans, looking out for each other, spending their days scavenging through the dumps and running away from fights. Even if the constant guard they had up was wearying, it was still them against the world, and Papyrus had felt secure in the knowledge that at the very least his brother was there for him, and would do anything for him.

But ever since they came here, things have changed. Sans has to go to the labs, insisting that Papyrus stay in their room and lying to Gaster that Papyrus doesn’t know how to read. He has to apparently assist Gaster with his experiments - whom Papyrus does not like from the moment he offered Sans a job. He doesn’t like the way Gaster looks at Sans, watchful and hungry, doesn’t like the way he had curled his hand over Sans’ shoulder when they were brought to their new accommodations, doesn’t like how Sans spends more time with him in a day than he did with Papyrus.

He misses those days, but Sans sleeps better with a lock on the door, and every time he looks so grateful that they’re both safe Papyrus can’t quite say anything.

So when they’re both tucked in and the lights are out save for the one in the corner, Papyrus pretends that he’s asleep. He keeps his eyes closed as Sans holds him tightly before getting out of bed, as he shuffles his way over to the door and goes out, locking the door behind him.

Something is definitely up. Sans has always valued his sleep. Papyrus breathes for five counts before scrambling out of the bed and after his brother.

He catches the tail of the lab coat his brother now dons on a regular basis disappearing around the corner, and makes his way silently over, just in time to see Sans knock on the door to Gaster’s private quarters. A voice calls out, “enter,” and Sans steps through the entrance into a darkened hallway. He proceeds to the end of the hall, into another room where light spills out of its door in a narrow beam.

The urge to call out to his brother is strong, but so is his need to know what has Sans so rattled, and why won’t he tell Papyrus? So he slides up to where he saw Sans entered, and puts his eye socket to the crack in the door left unclosed.

“You’re finally here,” the same voice from earlier says coolly. Papyrus nudges the door a tad wider, and he sees Gaster standing, tall and regal in his black coat with his hands behind his back, next to a bed much larger and more luxurious than the one they had been given. “I thought you might have been foolish enough to run, but it seems like you still retain some intelligence.“

“you’d just hunt us down if we ran.” Sans’ back is to the door, but Papyrus can hear every word his brother says, bitter and defeated. “i’ve seen the surveillance videos - you have eyes in every area. we wouldn’t last a day.”

“Correct.” Gaster smiles down at his brother, cold and satisfied. “So it seems that you have come to your senses.” A bone-white hand materialises mid-air and Papyrus watches, fascinated, as it dives down and tips his brother’s skull up. His brother flinches, but does not move.

“You will do everything I say?“

“yes.”

“You shall start by calling me sir.“

“yes, sir.”

“Very good.” Gaster pauses. “Strip for me.“

Sans remains immobile. Gaster sighs, and the hand slides up to cup the side of his brother’s face.

“Perhaps you require a reminder?” He suggests, his voice disaffected and bored.

“no.” Sans’ voice wavers. “i’ll do what you say.”

“Sir.“

"sir.” A deep breath in. “i’ll do what you say, sir.”

It’s a slow process. First the lab coat, white and pristine, pooling onto the floor. Sans takes off his shirt next, tugging it over his head. Throughout the motions Papyrus watches, as his brother’s ribs and backbones are revealed, his eyes catching yet again on the slender curve of his brother’s spine even if he has seen it all before, back when he had insisted on a dip in the cold waters in Waterfall. Sans had been adamant about keeping his pants on, just in case they needed a quick escape.

His brother reaches in front, and his pants drop to the floor.

This is wrong. There is something wrong, because this is the first time Papyrus can remember Sans entirely naked - and he looks so vulnerable, so small, hunched in the soft yellow light. Gaster’s own face remains impassive and unreadable.

Why is Sans doing this? He remembers his brother telling him that he should never leave himself vulnerable, especially if he refuses to fight - and here Sans was, casting away everything that could have been his armour. If Gaster decides to dust him right here and now, his brother wouldn’t even stand a chance, so why?

“Good boy.” A shudder runs through Sans, even though the room is nowhere near cold. Gaster apparently likes his quarters warm. “On your knees.”

(Sans looks even smaller on his knees.)

Another hand unfurls out of thin air, deftly undoing the buttons on Gaster’s black coat. The first hand leaves his brother’s skull and flies back up to assist Gaster in taking his coat off, revealing a grey tunic and black pants underneath. One step forward, and the skeleton’s pelvis is level with his brother’s face.

“Unbutton them.”

Papyrus’ hand grabs onto the frame of the door - he should be running in, stopping this. Sans shouldn’t, shouldn’t be doing whatever is happening, shouldn’t be reaching up and undoing the buttons and pushing the pants off Gaster’s hip - Sans should be sleeping in their bed, with Papyrus, and not spending time with Gaster.

Something purple glows, pulsing at the base of the taller skeleton’s exposed pelvis. Papyrus squints at it, trying to see and _oh_. Oh.

Papyrus isn’t stupid. He’s aware that at best he can be naive, but he knows about intimacy and their connotations. Even worse, he knows that it’s supposed to be done with people you trust, because baring the basest of your magic means that they aren’t likely to kill you.

Did Gaster trust Sans? Did Sans trust _Gaster_? What about Papyrus? Was Sans going to leave him for the first person who’d show him affection?

“Lick it,” Gaster orders.

To Papyrus’ shock, Sans leans in towards the glowing purple dick, obscuring it from sight, and does as he is told.

Sans doing as he is told. The thought inadvertently sends a thrill through Papyrus. To order his brother around, and his brother would do so, because it was Papyrus that said so…

Small gagging noises permeate the air, and Papyrus realises that Sans is now taking the cock down his throat, encouraged by the hands rubbing at his skull. Gaster himself is hunched over, whispering something to his brother, and his brother _whines_.

The whine sends a jolt up his spine. He wants to hear it again, he wants to make Sans make that particular noise again, wants to be the one to make Sans make that noise. Gaster has no right to come between them, not when Sans has always had Papyrus who has always had Sans. It’s not fair.

It’s not fair.

And perhaps for the first time, Papyrus thinks about dusting another monster. Maybe this is why the world is so cruel, because monsters want things that other monsters are taking away from them, and Gaster is taking away Sans from Papyrus.

Did Sans want this? _If he didn’t want this he wouldn’t be doing it_ , a small voice whispers in his head. Sans wants this.

Papyrus feels sick.

“Stop,” Gaster commands, sounding slightly breathless despite his composure. Sans pulls away from the cock, now glistening purple, and Papyrus can hear his brother’s wet gasping, unbearably loud in the silence. “Get on the bed.”

Sans scrambles to his feet, turning towards the bed - and Papyrus immediately shifts away from the door on instinct, just in case Sans sees him. But there’s no sudden outcry, no rushing towards the door, so Papyrus peers back into the room, assured of his undiscovered voyeurism. His brother sits in the centre of the bed, his head bowed, a line of spit running down the side of his jaw.

“Look at me.”

Sans looks up. His gaze is lost and broken.

(Papyrus wants that gaze on him.)

“Form me a cunt, and finger yourself.” The hands lifts a chair into Papyrus’ view, and Gaster seats himself. It does not block his view to the bed, and in effect his view to Sans, and for that Papyrus is relieved.

Why is he relieved? He should be disgusted. It’s disgusting, the levels Sans has let himself be debased to. Why is he listening to Gaster?

But Papyrus looks on anyway, looks as his brother’s magic coalesces at his pelvis, as his brother spreads his legs and slips a single finger in. It’s a slow and steady motion, how the phalange slips into the red folds of the summoned cunt, back out and back in again. Sans breathes out, a slow and long drawn out breath.

“Another,” Gaster tells his brother, and Sans complies, his breath becoming sharper and faster. Gaster is leaning forward, and Papyrus too is leaning forward, mesmerised by how the fingers are just sliding smoothly into the cunt. Sans is curling in towards himself, eyes closed in perhaps concentration, his posture kept open only by how his other hand is grasping at the ink black sheets of the bed. He gasps, small little breathy noises that betray his pleasure, his hips making small aborted rolls into the air and his toes curling into the side of the bed.

There’s a sort of tingling feeling at Papyrus’ own pelvis - and a quick glance down reveals his magic forming a cock similar to the one Sans had been licking earlier, bright red and hard. He touches it tentatively, grips it firmly, and wonders if Sans would -

Gaster suddenly stands up, the chair scraping across the floor, and Sans’ eyes fly open. The taller skeleton steps towards his brother, raising a knee and putting it in between Sans’ legs, and draws the hand that Sans was using to pleasure himself up to his own face. The two fingers his brother were using are coated in a clear, sticky fluid that slides slowly down the fine bone, and Gaster curls a purple tongue around them, taking them into his mouth.

The alarmed confusion on Sans’ face is soon twisted into pleasure as the floating hands pushes Sans down and spreads Sans’ legs apart by his thigh bones, as Gaster himself bends forward and licks into his cunt.

God, the noises his brother makes. All whines and moans and gasps as Gaster pins his pelvis down and mercilessly plunges his tongue into the opening, and a particularly loud cry when Gaster bites down harshly before resuming his licking. Papyrus’ own erection is pulsing as he roughly slides his palm against it; he wants to be the one pushing Sans down and eating him out, the one making him make all those noises. He wants to swallow those noises up into his mouth and down into his ribs where he can keep them as his and his alone.

“stop,” his brother is saying, is begging, “stop, i don’t,” but his hands are digging hard into Gaster’s shoulders, as if he is pulling Gaster towards himself rather than pushing him away. Gaster gives the cunt one last lick with his tongue and looks up at Sans, almost considering.

“Do you really want me to stop,” he asks, pushing himself up and boxing Sans in. Sans just seems to shrink into the bed with Gaster looming over him. “You’ve been doing so well,” and Papyrus doesn’t miss the faint flush of pleasure on his brother’s skull. “And you don’t want to stop, do you?”

Silence reigns for a second.

“no, sir,” Sans whispers.

"Good boy,” Gaster praises, a hand stroking down and over the curve of his brother’s skull. “Now give us a kiss.”

The ensuing kiss that happens is less a kiss than Gaster shoving his tongue down his brother’s throat as Sans lies back placidly, eye sockets wide open and unseeing. The scientist draws back, his countenance twisting into irritation, and stands back up. Sans sits up and looks back at him, in an expression that hints at the beginning of defiance - and Papyrus begins to think that perhaps there’s something else going on, maybe his brother isn’t -

“If you do not want this,” Gaster says softly, cutting across Papyrus’ train of thought. “We can stop.”

 _Yes_ , Papyrus thinks furiously at his brother. _Stop_.

The scientist leans in, and Papyrus does not see the smirk he wears. “After all… there are other beds I can grace my presence with tonight.”

 _So do that_ , Papyrus thinks, _leave Sans to me,_ but Sans is suddenly surging up, his hands pulling Gaster’s skull in, their teeth meeting in a harsh clack. His brother fervently kisses Gaster, squeezing his eye sockets shut, allowing Gaster to lick into his mouth and making muffled noises into the kiss, and Papyrus feels… he feels…

He feels betrayed.

(Yet still he cannot look away.)

Gaster presses a clothed knee to Sans’ pelvis, rubbing the material roughly against the still-dripping cunt. Sans gasps into the kiss, his body jolting forward - and the hands shift to hold him in place, to allow Sans to rut into Gaster’s thigh, to allow Gaster to drink up all the moans. His brother clutches at the fabric of Gaster’s shirt like it is a lifeline, and as much as Papyrus wants to tear his hands off the other skeleton, he can’t.

So Papyrus clenches his jaw and jerks himself off in slow but sure strokes, imagining that instead of Gaster it’s Papyrus kissing his brother, it’s Papyrus grinding his knee into his brother’s sensitive pussy, it’s Papyrus who is now manhandling Sans to sit on his lap as he himself settles against the plush cushions at the headboard before he finally lets Sans go, a gasping and panting mess.

“Good boy,” Gaster purrs, and Papyrus imagines how Sans shudders against him, their sweat-slick bones slotting into perfect places.

They stay in place, Sans catching his breath and Gaster allowing this brief reprieve before saying, “you know what to do.”

Another breath.

“yes, sir.”

His brother places his hands on Gaster’s shoulders, rising up and positioning himself over Gaster’s cock, and lowers himself. A sharp breath in as the head brushes against the folds, and Sans’ bones begins to tremble and rattle against each other as the cock penetrates him deeper and deeper, until he finally falls forward into Gaster’s chest as he seats himself fully.

“Well?” Gaster says. His many hands twitch, but stay in place.

Sans’ breath hitches just the once.

His brother begins to raise himself back up, his entire body shaking, before sliding back down onto the cock, riding Gaster into the bed. The pacing is slow, so slow, and Papyrus wants to grab Sans’ hips and slam him down, to drive his own cock into Sans’ opening.

It seems like Gaster has the same idea, because the two hands flies down and grasps Sans by his pelvis. Sans’ eye sockets fly open, and he opens his mouth to protest, “n-”

The hands pushes his hips down, a swift and final motion, and Sans cries out.

“That’s more satisfactory,” Gaster laughs, a pleased gleam in his eyes.

The hands lift Sans up again, and pushes him back down, and all the while Sans is writhing in Gaster’s lap, trying to twist away from the punishing pace that the hands have set. Gaster’s own hands reach up into the empty space of Sans’ rib cage, his fingers dragging across the rows of bone, and Papyrus’ own hands move faster to the tempo set by his brother’s voice, a symphony of ‘ah’ and 'please’ and moans.

“Say my name,” Gaster encourages in between breaths, sharp fingertips scraping down the length of Sans’ spine. His brother whimpers into Gaster’s shoulder. “Say my name, Sans.”

It takes several tries, across several thrusts.

“gaster.”

“Louder.”

“g-gaster.”

“Louder.”

“mnf- ah- _gaster_ -”

And in that moment, Gaster looks up, past Sans, past the door and directly at Papyrus.

Papyrus freezes.

How long has he known, what is he going to do, how dare he -

Gaster smiles, wide and shark-like and all too knowing, holding Papyrus’ gaze as his arms circle around Papyrus’ brother, as he opens his mouth wide and bites down onto the curve of his brother’s collarbone, as his brother screams in pain and pleasure -

A hand unfurls right in front of Papyrus and shuts the door in his face, leaving him alone in the dark.

But the walls and the doors of this complex were never meant to be soundproof - so Papyrus hears every drop of praise Gaster feeds his brother, every sob and moan his brother makes, every word his brother says, a litany of _gastergastergaster_. His imagination supplies the rest - how he would tip his brother over onto the bed and replace that name with his own, how he would hold his brother down in the dusty remains of that bastard, how he would leave his own marks to tell the world that he had Sans first and no one else.

He gives his cock one final vicious twist of his hand, and comes to the sound of his brother crying a name that isn’t his.

(Later Papyrus will pick himself up and return to their room, and stay awake in the newfound knowledge that perhaps he will never be as good as he wants to be.

Or perhaps he will never be as good as Sans wishes for him to be.)

Sans does not return to the room that night. In Papyrus, the beginnings of resentment start to simmer.


	3. wake-up call

**Notes for the Chapter:**

> thanks for all of you who left comments. tbh there were times i thought that maybe i should have let the story end there - but i think uf!paps would kill me if i did. 
> 
> thanks for sticking by me, and let's get this shitshow on the road.
> 
> note: small edit made to chapter one, but otherwise minor.

The first time they were caught in an ambush, Papyrus froze.

He doesn’t remember who or what or why - it might have been the gang they had been skimming food off from, with their supplies in plain sight; or it might have been some opportunistic monster looking for the next rungs to add on their ladder of reputation. One moment he saw a body barrelling towards him - and the next Sans was shaking him aware in some cave, eyelights aglow and desperate.

“papyrus, please,” he had said, the red wisps of magic dissipating to reveal intense relief on Sans’ face, before -

“papyrus - what the hell were you thinking? why didn’t you fight back? you could have been dusted!“

What was he thinking, he doesn’t remember, but he remembered that they had been practicing with their magic just hours before. Papyrus was getting a bit too big and heavy for Sans to carry and run away, as they had done when Papyrus was still a baby bones. He needed to know how to defend himself, just in case, and Papyrus just - Papyrus just -

“I’m sorry,” he had blubbered, tears welling up in his eye sockets, “I didn’t want to hurt anyone, I, I - ”

And Sans had immediately comforted him, telling him it’s okay, he doesn’t have to hurt anyone, because Sans, as soft-hearted as he actually is, always gave in to him. He could never stand to see Papyrus cry.

And in that moment, Papyrus thought, _how lucky I am to have Sans_.

* * *

In this moment, Papyrus is furious.

 _It’s not fair._  The thought rages through his mind. Gaster has already taken so much of Sans from him: his time, his attention, and now he’s stealing Sans’ affections. He takes and takes and takes and Papyrus doesn’t know what is left of Sans for him to hold onto.

_It’s not fair._

How did this even happen? Perhaps it’s gratitude - after all, Gaster had saved them. He had given Sans a job, a home, a bed - everything that Sans could ever want and Papyrus could have never given him. All Papyrus has ever brought him were empty words, of his hopes and dreams of a better world. In that world, monsters were nice and no one dusted each other. In that world, they were both safe and happy.

That world was crumbling away. It doesn’t matter if Papyrus is safe, if all the monsters in the Underground gives him candy and otherwise leaves him alone, if Gaster took Sans away from him.

In his mind he can see Sans approaching Gaster, eye sockets wide and adoring. Perhaps Sans made the first move, his hands reaching up, pulling the other skeleton in for a brash yet tentative kiss. Or perhaps Gaster had beckoned his brother and Sans had followed, soft and compliant under his touch.

His bones are rattling, his entire body shaking in pure anger as something dark and bitter curls and snarls from deep down in his gut. He thinks of Sans shuddering as a purple tongue runs over the dips in his spine, curling at the very base of the coccyx; he thinks of Sans panting softly, muffling his cries by biting into his arm.

He _remembers_ Sans moaning, deep and dirty and so very loud.

(This is what Papyrus refuses to think of: pale bones in the dim light; the tip of a skeletal finger skimming down the inner curve of a rib; his brother splayed out on the bed, spine rising in a perfect arch.)

This is about trust, he reminds himself, willing his body to stop shaking. It was about how his brother trusted this skeleton he had just met over Papyrus, who had been there for him all his life. Sans had refused to bare his bones in front of Papyrus, and would do so in front of Gaster; he had stripped himself vulnerable while Gaster had remained clothed.

It was about how Sans was abandoning him after all they’ve been through. A taste of a life better than what they had, and Sans was already throwing himself at his benefactor.

 _Disgusting_.

(Sans reaching up to him, reverently whispering, _papyrus_.)

He could be doing the very same thing right now, Papyrus realises. For all Papyrus knew, Sans’ job of ‘assisting’ Gaster could very well be a lie. Sans could have been patronising him with all his talk about _science_ and _experiments_ \- and Papyrus had accepted it like the naive little fool he was.

He’s going to put a stop to this. Sans isn’t allowed to leave him for Gaster, because Papyrus won’t let him. He’s going to find Sans, and they were going to leave this place. Once Gaster is out of the picture, Sans will realise how _embarrassing_ he’s been acting.

Papyrus stands up and opens the door, taking a step out of the very room Sans told him not to leave. He does not look back.

* * *

“not every monster is going to be as nice as you want them to be, paps,” Sans had told him once, later, perhaps as an attempt to teach him better.

 _I only need you to be_ , Papyrus had not said then.

* * *

The corridors of the complex are long and winding. He walks down one and turn into another that looks almost identical - but Papyrus knows where to go.

All he has to do is follow the screams.

(The screams had been unsettling, at first - but he had grown used to it over time. Sans had told him that they were criminals put to death by the capital. Papyrus had told himself there was nothing he could have done.)

It takes a certain amount of stealth, and an overwhelming amount of luck - but years of living on the streets has given him an advantage. He darts from door to door, hiding in the shadows and skirting corners when the coast is clear, always keeping an eye out for harried scientists and irate guards. None of them are likely to tell him where he could find their boss, much less when he’s the younger brother of the newest employee.

He wonders if they even know that their colleague has a younger brother he hides away in his room. Or perhaps Sans spends the whole day around Gaster, at his beck and call, his time occupied by being Gaster’s ‘assistant’. His brother wouldn’t have the knowledge nor expertise to help the other scientists out on their experiments, after all.

All the pieces of the puzzle he had never wanted to solve are fitting into place - all pointing towards this sham of a job. Why would the Royal Scientist, impossibly brilliant and favoured by the king himself, want a street rat to be his assistant?

Well, now he knows why.

Adrenaline and conviction pulses through his marrow, pushing him on - up until the screaming abruptly stops, in a way Papyrus knows to mean that the unlucky monster had been put out of their misery.

But in the deafening silence, Papyrus no longer knows where to go. For the first time, he thinks that maybe he should have stayed in the room, waited for Sans to come back before confronting him -

(Gaster smirks at him, smug and entitled as he fucks into Papyrus’ brother.)

No. He had to find Sans now. He wouldn’t know how to get back to their room anyway, not without the help he cannot ask for.

He takes a step forward, ready to leave his chances up to luck - when suddenly, a pale white blur flies by him, a stark splash of magic against the dull interior of the complex. It stops next to a door, hovering in place long enough for Papyrus to recognise the perforated hand he had seen last night, the one that had held his brother down and slammed a door in his face.

The hand picks up a clipboard from an open tray, and continues down the hallway.

Papyrus follows.

It sets a face pace, leading him down several more corridors before halting in front of an otherwise nondescript metal door and setting the clipboard down on a nearby table. Papyrus watches from around the corner as the hand knocks sharply twice on the door before retrieving the clipboard. The door swings open and it flies in; the door swings shut.

Gaster is in that room. Sans is likely to be there too.

He strides up to the door and knocks twice, a sharp rap-rap just like he had seen. The door swings open.

Papyrus steps through. The door swings shut.

* * *

He finds Gaster.

The royal scientist stands before him, perusing the clipboard Papyrus had followed. The mere sight of the other monster sets him back on edge, fury rushing and roiling through him like a heatwave. He remembers the way Gaster had marked Sans, all the while looking at Papyrus and taunting him over what he didn’t have - and the urge to rip Gaster’s teeth out and grind his skull into dust returns in its full intensity.

Papyrus had previously considered himself above hate and holding grudges, but this feeling is best described as pure _loathing_.

But he’s not here for Gaster, as much as he wants to snap the other skeleton’s neck into two. He’s here for Sans -

Who is nowhere to be seen.

All his plans come screeching to a halt. He had expected Sans to be shadowing the person he was supposed to assist, but had not really considered what happened if he found only Gaster. Where else could Sans be, if not with Gaster?

“Finally left your cage, I see,” Gaster says.

His attention snaps back to Gaster, who is still looking down the clipboard, as if Papyrus was someone not worth paying attention to. The notion irks him.

“Well?” The scientist prompts.

“Where is Sans?” Papyrus demands.

“He’s working,” Gaster replies dismissively, still not looking up at Papyrus.

“ _Where_?“

“Do you need him for something?” A rustle of paper as Gaster flips over a page, and Papyrus doesn’t have the patience for this, for the other’s nonchalance and disregard.

“We’re leaving,” Papyrus declares.

The finger skimming down the clipboard stills for a split-second.

“Today,” he adds.

“Is that so,” Gaster says, eyes still trained on the clipboard. “I suppose Sans knows about his immediate resignation then?”

Papyrus hesitates. The other skeleton finally, finally looks up, and a smile spreads, tar-slick across his face.

“He doesn’t know?” A hand flies to retrieve the clipboard, removing it to the far side of the room. Gaster stares at him, eye sockets wide in a mockery of surprise. “My, my. Making decisions for each other? How very… _brotherly_ of you.“

Papyrus bristles.

“He’ll understand,” he says brashly.

“I’m sure he will,” Gaster agrees. “Like how he thought you would when he took this job.”

“I - He thought we needed help,” Papyrus says, because situations change, they really do. “We don’t. We don’t need _your_ help.“

“You don’t,” Gaster says. “Sans, on the other hand…” He pauses for a moment, before saying, “you’re awfully selfish, aren’t you?”

Selfish? He was doing this for _Sans_.

“I’m not selfish,” Papyrus scowls, hating the way his voice turns petulant, like a kid who didn’t get what they wanted.

“But you are,” Gaster says, almost as if Papyrus’ words didn’t make a difference. “What are you going to do when you go back out there? Let big brother protect you again, while you wait and watch from the sidelines? Even though he’s so weak - “

"Sans isn’t weak,” Papyrus bites out, his hands curling into fists.

“And you just let him,” Gaster continues. “You could have fought back, helped each other survive - but no, you were content to hide behind big brother and let him face all the danger all by himself. Let him dirty his hands for you, while you preach at him about goodness and kindness.”

“No,” Papyrus says, but his words are catching in his throat, refusing to come out. How did Gaster know - but no, it didn’t matter how he knew, because he was right. After that ambush, Sans had fought harder, defending both himself and Papyrus, never relying on his little brother.

“No,” Gaster repeats softly, relishing every word. “No, you let him fight even though one hit could have dusted him. Even though you were stronger than he was. All for your pretty little morals.”

“They’re not,” Papyrus says, “they’re not little, they -“

"They made you a _burden_ ,” Gaster says languidly. “It’s comfortable, isn’t it? Having someone to follow you, to agree with whatever you say. To take care of you, and you don’t even have to lift a finger.“

“But it’s tiring,” and here, his words turn sharp and cruel. “Sans got tired.”

“Shut up,” Papyrus snaps. It’s not like that, it won’t be like that, he sees his mistakes now. He just has to prove it to Sans.“Y-you’re wrong.“

“You don’t have to believe me.” The hand returns, another clipboard in its grasp. Gaster takes it, turning away to read through its contents; a dismissal, like Gaster had imparted his attention and free time onto Papyrus on a mere whim. “I certainly won’t stop you. There are plenty of monsters who would want to work under me.”

Papyrus opens his mouth, if only to correct Gaster - but suddenly there are three quick and sharp raps, and the whining creak of the door he came in through.

“Looks like your little adventure is over,” Gaster comments, just as Sans steps into the room.

* * *

The first thing he sees is the dishevelled state of Sans’ lab coat, creased and folded.

 _Like it had been left on the floor_ , his mind reminds him, a sibilant whisper.

The next thing he sees is the way Sans’ eyelights flicker from Papyrus to Gaster, a whirlwind of emotions cycling through his face. Bewilderment, shock, fear and then panic - then Sans is suddenly in front of him, pushing Papyrus behind him and blocking him from Gaster.

“you asshole, you said,” Sans snarls, but Gaster interrupts him.

“I did nothing,” the scientist says, calm and composed, and Papyrus can peer from over the top of Sans’ skull how he’s placed the clipboard down onto the table. When had he grown taller than Sans, he absently wonders. “Your brother came to find you.“

A brief moment of tension, and Sans’ grip on him grows ever tighter.

“It’s true,” Papyrus says, and Sans’ skull jerks back to look at him. “I needed to talk to you.”

“you could have waited in the room,” Sans says, “i told you to stay in the room, papyrus -“

“It’s urgent,” Papyrus interrupts, looking straight at Gaster.

Gaster only smiles.

“Why don’t you return your brother to your quarters, Sans,” the scientist suggests. Papyrus doesn’t miss the way Sans immediately tenses. “Unless you would like him to observe?”

“no,” Sans says, sharp and almost panicked. “no, i - come on, papyrus. let’s go.“

And even though Sans is taking his hand and leading him out of the room; even though they’re leaving Gaster behind in that cold laboratory; even though by all rights Papyrus has Sans with him right now -

Somehow, Papyrus feels like it’s him that has lost.


	4. communication is important but you still fuck it up

**Notes for the Chapter:**

> shoutout to the usual bunch: sympatricuckoo, beescream, 0netype
> 
> check out i gave an inch fancomics and fanarts AND fansequels at the sinfic masterpost [here](http://sweetsinnerchild.tumblr.com/sinfic-masterpost)
> 
> thanks for stickin' around, and i hope you stay for more bad times
> 
> Note: i finally got myself an ao3 account so check out the other stuff i wrote, now no longer under an anonymous author! ayyyyy

The door swings shut.

“did he do anything to you?” Sans immediately asks, frantically checking Papyrus for any visible injuries. “did he hurt you? did he -”

Because even though Gaster did say he did nothing, and even though Papyrus did agree with him, Sans couldn’t quite shake the notion that he didn’t quite have the full picture, that something was different. Perhaps Gaster could have said something to him, could have told him to do something, could have told Papyrus about how his older brother was so weak and so easy he -

( _\- he lies limp in Gaster’s bed as the other skeleton rocks into him, slow and languid. The bitemark on his collarbone throbs, a dull ache that temporarily sharpens with every thrust. He focuses on the ebb and flow of pain to block out the motion below._

 _If he closes his eyes, maybe he could pretend he was asleep_.)

“He didn’t do anything, Sans,” Papyrus says. “I told you, I came to find you.”

“you should have waited in the room, i told you it was dangerous out here,” but there’s a sense of relief, that Papyrus doesn’t know. Maybe Gaster is telling the truth - after all, what would he have to gain by telling Papyrus? Knowing his brother, Papyrus would only…

What would Papyrus do? He might demand for them to leave, to get out while they still can, damn the consequences. _I’ll protect you, brother,_ he might promise, ever so sincere. Always putting others before himself.

( _The bed shifts under him as Gaster leans down. Sans stiffens as a tongue laves across the mark, over and over, as if the other monster was soothing him; and he doesn’t want this. He doesn’t want the imprint of Gaster’s teeth on his clavicle, doesn’t want any reminder of tonight. He needs to leave._

_As if he had read Sans’ mind, Gaster stops, turning his head into the crook of Sans’ neck._

_“I expect this mark to be here next time,” Gaster says clearly, before returning to his task_.)

“But it’s urgent,” Papyrus says, but Sans is thinking - they couldn’t leave, not with Gaster coming after them and doing everything he had promised Sans he would do.

He imagines Papyrus sending out shakily formed bullets.

He imagines Gaster striking him down.

He can’t tell Papyrus. He can’t risk his brother ever setting a chain of well-intentioned actions putting the both of them in danger, with neither of them able to fend themselves from one of the most powerful monsters in the Underground.

Most importantly, he doesn’t want Papyrus to know how pathetic his older brother truly is.

( _Gaster pulls out of him, finally sated. He turns away, and Sans pushes himself up and out of the bed, ready to leave._ It’s over, _he had thought, empty relief pulling at his heart. He could finally return to the privacy of a dark and empty corridor to fall apart with no one the wiser._

_Sans takes a step towards the door, even as his knees threaten to buckle, because all he needs to do is leave. He needs to leave._

_“Where do you think you’re going?”_ )

He can do this. He has to do this, pretend everything is alright, pretend that letting his boss fuck him is all part of his job. For Papyrus, he has to-

( _Sans freezes._

_Hands materialise around his wrists, tugging him back onto the bed, settling him down. An arm curls around his chest, effectively pinning him to the body next to him._

_“please,” he finally says, begs, choking on the tears threatening to overflow because he can’t, he can’t, he can’t do this -_

_“Stay,” Gaster murmurs_.)

He has to -

( _Sans cries himself to sleep_.) 

* * *

“SANS!”

He’s pulled back by Papyrus’ sharp voice, back into the reality of his present. Papyrus’ frustrated countenance greets him, almost as if he had been calling Sans for some time now.

“I said, I needed to tell you something,“ Papyrus is saying, before frowning. “Sans, are you listening? Sans?”

“paps,” he says. “paps, i - “

He wants to hug his brother so badly. He wants to have someone touch him without asking for more than he can give. He wants Papyrus to reassure him that affection can mean more than simply taking and leaving nothing but the feeling of phantom hands on his body.

He wants, and yet.

“Sans,” Papyrus says, almost uncertain.

(He has to.)

"sorry, paps,” he says, forcing a smile, paper-thin and shaky, forcing his shoulders down from its initial hunched posture. “i’m just a bit rattled today. bad experiments.” It wasn’t a lie. “what, what were you saying?”

Papyrus hesitates. Sans immediately begins to worry - Papyrus did say that it was urgent, back in the labs with Gaster. Perhaps something serious had happened, back in their room - and Sans had been caught up in his own pathetic self-loathing to care.

But before Sans could say anything to fix his mistake, his brother’s face hardens with resolve.

“I want you to know that I love you.”

The words are firm yet tender and entirely unexpected. Whatever Sans had thought to say dissolves with those words - partly from confusion and partly from warmth. Papyrus steps in closer, bringing his arms up to hug Sans, and Sans immediately hugs back tightly, luxuriating in the contact.

Trust Papyrus to know what he needs, Sans thinks gratefully as he sinks into the feeling of someone just holding him, even if Sans never said a word.

“I love you, Sans,” Papyrus repeats, over his uninjured shoulder. “I really do.”

Sans breathes out, almost a nervous laugh. “what brought this on, paps? you came all the way to tell me that?”

Papyrus’ arms tighten around him. It would remind him of Gaster, but no. Sans is safe with his brother.

“You love me right,” Papyrus asks, burying his head into Sans’ lab coat, muffling his words. “Don’t you?”

That was never a question.

“of course i do.” He feels Papyrus smile into his shoulder, as he brings a hand up to stroke his brother’s skull. “you’re the best brother i could ever ask for.”

For a moment, he thinks Papyrus freezes.

Then Papyrus pulls away from him, looking at him, that same resolve etched on his face.

“I need to tell you something,” Papyrus says. "I want us to leave.“ 

* * *

Sans had wanted to leave, once.

When Papyrus was a baby bones, he was a loud crier. Maybe he knew then that this world would not pay attention to him if he did not force it to, or maybe in some innate sense he knew that no one was there to do so in the first place. But Sans had been there to calm him, something that Sans himself did not have the privilege of when he himself had been younger.

Staying quiet was the way to survive, and Papyrus was…

He was a child. He couldn’t feed himself, couldn’t run away quickly enough, didn’t know enough about stealth to realise that crying made you look vulnerable, made you look weak. Sans couldn’t blame Papyrus, but he could resent him for the way he made Sans do everything for the both of them. He could hate him for how he ate everything Sans barely managed to get his hands on and asked for more, for how he repeatedly gave away their location with an ill-timed tantrum.

So he had told Papyrus to stay in a cave, hidden behind a waterfall one day. Told him that Sans would be back, to sit tight and wait. And he would run, run far away where Papyrus couldn’t find him but Sans would be fine. He’d be fine without his brother.

So Papyrus stayed, and Sans went.

The first day passed. He went into Snowdin, skirted the edges of the town and when night fell he dug into the trash. A half-eaten burger, some fries and a torn packet of ketchup later, Sans sat on the slope of a snowy hill… and…

Did nothing.

It was quiet. There was no one pulling at his sleeve, loud and insistent.

It was relaxing. There was no one to hush lest they be caught.

It was lonely.

(There was no one, no one to hug, no one to talk to, no one who cared about him.)

Sans was so, so alone.

He went back the next day. Papyrus greeted him with a smile. 

* * *

The world seems so very far away.

"leave,” Sans finds himself repeating.

“Yes, leave,” Papyrus is saying. “I don’t like this place, Sans. I want to go back to Waterfall.”

 _Back to Waterfall_. Back to running and hiding and fighting, to stealing their meals and hoping to live another day. Back to where Gaster had cameras in every area, and could track them down and….

“what’s wrong with this place, paps,” Sans asks, pleads. _please change your mind_. “isn’t it safe here? did someone hurt you?”

“No,” Papyrus says, “but I don’t want to be stuck in a room without knowing where you are, Sans. I’m worried.”

“but it’s okay, i’m safe,” Sans tries to assure him. “i’m just working, papyrus. they’re not doing experiments on me.”

“But they _could_ ,” Papyrus insists. “They could just lock you away and I wouldn’t know any better.” He tugs at Sans’ lab coat, as if he’s trying to pull him over to Papyrus’ metaphorical side. “Out there I know where you are.”

“but,” Sans says.

“Don’t you love me, Sans,” Papyrus says, beseechingly. “You said you loved me.”

“i do, i do love you,” Sans says, and that’s why they have to stay. “paps, i’m… we can’t leave.”

“Why not?”

 _Because you’ll be tortured if I take a single step out of this place_ , Sans thinks hopelessly. _Because I’m too weak to fight back and too much of a coward to try._

“i… i can’t protect you.” That is true. “i’m not strong enough.” That is also true.  
Papyrus draws back. Sans feels the emptiness in his arms; he regrets.

“You’re tired,” his brother says, his voice strangely hollow.

"paps…”

“I can protect us for you,” Papyrus says, his words coming out in a rush of desperation. His hand clenches on the sleeves of Sans’ coat. “I - I promise I won’t be a burden on you.”

“that’s not - papyrus, you aren’t a burden - ”

“Then why?!”

(A single word, a single command. “ _Stay_.”)

“i…” He looks away, hopes that Papyrus cannot see the shame in his eyes. “i can’t leave, Paps. i… just can’t.”

A moment of silence. Sans studies the wall, looks at the door, looks at anything that is not his brother. For a moment he hopes that Papyrus understands, that Papyrus will give up. 

“You just want to stay with him,” Papyrus finally says.

Sans’ head snaps up. Papyrus stares at him, mutinous and angry, lower jaw trembling subtly. Papyrus, who had been hugging him mere moments ago, who told Sans that he loved him. He almost doesn’t recognise the monster in front of him, so angry, so bitter.

“papyrus…?”

“You just want to stay with him,” Papyrus repeats, his voice low and controlled, the way a river is controlled before the dam bursts. “Isn’t that right?”

“with who -”

“ _Gaster_ ,” Papyrus all but snarls.

It feels like someone threw a bucket of ice cold water at him then and there, chilling him to his marrow. It feels like someone punched him in his heart and dug their claws in, dragging out all the warmth he had left. It feels like someone was pulling, pulling, pulling at him, at every part of him until there’s nothing left to be found.

“that’s not true,” his mouth says.

“Liar,” Papyrus immediately retorts, without a single ounce of hesitation, no deliberation required. “You spend so much time with him - ”

“that’s because i have work - ”

“You sleep when you get back - ”

“because i’m _tired_ \- ”

“I saw you with him!” Papyrus shouts. “I saw you _fucking_ him!”

Papyrus saw.

(The world slows to a stop.)

Papyrus _knows_.

“I saw you,“ Papyrus repeats. Sans feels the way his brother’s eyes bore into him, stripping away the bones and leaving only the shame underneath. “I saw you go to his room- and I heard you. The both of you.”

“it’s not what you think,” Sans says, still feeling sick to his stomach. Papyrus knows, he _knows_ , and but he doesn’t know the whole picture. He feels the way his hands shake and balls them into fists to stop them from betraying him.

“papyrus, it’s - 

“I thought we were brothers,” Papyrus interrupts him hotly. “You said you’d always choose _me_ over anyone else.“

“i do, paps- “ Sans reaches out to his brother, wanting to make him understand. “i always - ”

“Then leave with me!” Papyrus yells.

He should tell him, tell him what Gaster threatened, tell him why they can’t leave. Why he’s doing all this. Maybe Papyrus will understand.

(Dust on the floor, and he sweeps it all away.)

“we can’t,” Sans finally says, forcing steel into his voice. “and that’s final.“

Papyrus snaps. In retrospect, perhaps Sans should have known that in the years that his brother was kind-hearted, he had only managed to be so due to his stubbornness.

“You can stay with him then,” Papyrus lashes out, his voice bouncing off the corridors. “You’re not even a real scientist anyway! He probably kept you around to be his _whore_ \- ”

 _Slap_.

There’s a slight stinging pain in his left hand. He watches as Papyrus silently brings his own hand up to touch his own face, to touch the cheek where Sans had slapped him.

Sans had _slapped_ him.

“p-papyrus- “

The mention of his name jerks Papyrus into motion. His brother steps back, away from him, away from his outstretched hand.

“Don’t touch me,” Papyrus snarls. Sans recoils. “I’m leaving, I- I’m,” and oh god, his brother’s blinking back tears, because Sans had slapped him. He slapped him. “I… I hate you. I hate you!“

His brother turns and runs, runs away from Sans, down the corridor and round the corner and out of sight. Sans stands, wrecked by indecision - because he should go after Papyrus, it’s not safe but Papyrus hates him now. Papyrus _hates_ him.

(The hysterically funny part is that what Papyrus had said is _true_.)

He has to apologise, he has to explain, has to stop him before Papyrus leaves. But would Papyrus listen? Sans had hurt him. Papyrus probably doesn’t want to see him ever again. But - he’ll never see Papyrus again if he dies out there because Sans wasn’t there to protect him.

“papyrus, wait!” Sans finally finds his voice, taking a step forward, then another -

* * *

“Well, that was _fascinating_.”


	5. negotiations (with your back against a wall)

**Notes for the Chapter:**

> shoutout to sympatricuckoo and 0netype, as per usual
> 
> one or two more chapter until the end of this arc, if i can even write them hahahhaahhaaa haa
> 
> sorry for not replying to your comments, i see 'em and love 'em but i'm just like 'how to social'

Sans freezes. 

He doesn’t need to turn around to know who’s there. His bones are rattling, just like back when he was a baby bones, and he loathes himself for being scared. He loathes himself for stopping when he should have run. 

But his feet are cemented to the floor, unwilling and unable to move through no one’s fault but his own. He stays rooted in place as calm, measured footsteps draws close behind him, as a hand drops heavy on his marked shoulder, pressing down against the wound its owner left behind. It stings, a sharp burn that makes his body jerk away from the sensation; but the hand stays firm. 

“Wherever are you going, Sans?” Gaster inquires blithely. 

He does not, cannot answer, words lodged in his throat like a too-large piece of monster candy. How much had Gaster heard? Probably everything. He should have brought Papyrus back to the room before they had their fight. He should have been smarter, should have been quicker, should have ran away the very moment Gaster made his true intentions known. 

“Sans,” Gaster says again, his hand tightening, pressing more firmly against the shoulder. Sans bites down a cry, his shoulders hunching from the sharp stabbing pain. He hadn’t dared to heal his shoulder, not with Gaster looming over him the whole day. 

Why is he so afraid? All Gaster did was fuck him. He’d known that Gaster was dangerous, is dangerous, but he had been fine before. Why is he so damned _afraid_?

“nowhere,” he says, but it sounds less like a word and more of a rasp. He tries again. “n-nowhere. i’m not going anywhere.”

“Really,” Gaster lilts. Sans doesn’t look at him, keeps his eyes trained on the ground. “What about your brother?" 

God, Papyrus. Papyrus, who was going to leave, who can barely protect himself because Sans was the shittiest brother in the entire Underground. He has to get him back, because the laboratory is safe for Papyrus, if not for Sans. 

Nowhere is safe for Sans. 

"he,” Sans says. “he, he’s throwing a bit of a tantrum. i just need to talk some sense into him, real quick -" 

"He’s leaving, isn’t he,” Gaster states. 

“he wants to,” Sans hurriedly corrects Gaster, “i just need to -" 

"Sans,” the scientist says, voice silky smooth. “If you take one step out of this complex, I will be forced to consider our agreement void." 

And Sans feels another wave of dread sweep over him, chilled and numbing. 

"what do you mean,” he stutters out - but he knows. All the threats of experimenting on Papyrus, making him go through unimaginable pain, making Sans watch on helplessly as his baby brother suffers - 

\- making Sans watch on helplessly as his baby brother dies out there without Sans to protect him. 

“no,” he bursts, turning around to grab at the lapel of Gaster’s coat, his hands crushing the fabric underneath his grip, “please, i’ll come back, just let me convince him - " 

"You couldn’t even convince him just now.” Gaster smiles down at him, the curve of his smirk nothing but chilling. “Do you take me for a fool, Sans?" 

"no -" 

"If I let you go out there,” Gaster continues, “you two could very well run away. Let’s face it, it would be a very short chase - but you’ll be wasting my time.” His smile turns cold and amused, like that of an indifferent god. “I don’t like wasting time." 

"i know,” Sans says, begs because Papyrus is on the line, “but i’ll come back, please. i _promise_ i’ll come back." 

Gaster tuts, and shakes his head. 

"Promises, promises." 

"i,” Sans says, grasping at straws for how low he can grovel before Gaster can be satisfied. “please -" 

Maybe there’s a way he can convince Gaster, he thinks wildly. A favour for a favour. 

(Gaster’s eyes on him, hungry and wanting.) 

His mouth is dry. His arms are shaking. Every second he delays, Papyrus gets further away. 

"i,” Sans says, but the words are stuck in his throat, claws dug in sharp to prevent them from coming up. “i’ll. i’ll let you." 

"let me,” Gaster repeats. 

_let you fuck me again_ , Sans wants to say. _i’ll let you rape me again_ , but the words won’t leave his throat because deep inside, he doesn’t want another night of sharp pain and unwanted pleasure. He doesn’t want another incident proving Papyrus right. 

(Nevertheless.) 

Sans swallows, the unspoken words dragging shards of shattered glass down his throat. Slowly, carefully, he lets go of Gaster’s coat and takes a step back. The other skeleton merely watches him, like how he would when one of the test subjects manages to escape - not in concern, but with an aloof interest in what will the doomed monster do next. 

His hand grabs the collar of his shirt, and pulls it to the side, exposing the mark that Gaster had left last night. 

“i’ll let you,” Sans says again, and hopes the message gets across. 

* * *

In another part of the facility, Papyrus is running. 

He finds the exit by retracing his steps to the entrance from which they entered, weeks ago. It stands open, an open invitation to freedom. His hands tugs nervously once at the scarf around his neck, a tattered scrap of red cloth that his brother once wound around his neck back when it was them and only them. 

He doesn’t hear his brother behind him now. 

Papyrus steps out into the oppressive heat of Hotland, and runs. 

* * *

Gaster laughs. 

The sound echoes off the metal corridors of the laboratory, loud and sonorous, mocking Sans on all sides. Sans stands, his hand still on his shirt, baring his clavicle to anyone who cares to look. The only person to whom it matters is laughing at him. 

Maybe he understood wrongly. Maybe Gaster just wanted him once, just for the sake of it, and Sans had merely presumed that he wanted to do it again. Maybe Sans wasn’t as satisfying as he thought he was. The thought liberates him, because maybe Gaster will never look at him in a sexual light ever again. 

The thought eviscerates him - because now he would have no leverage to convince Gaster with. 

He lets go of his shirt, a flush of heat setting into his bones - only to feel the cold grip of Gaster’s magic wrap around his soul. A breath, more of a gasp - and Gaster slams him into the wall. 

He’s suspended midair, feet dangling and straining to touch the floor as the scientist pins him in place, like a butterfly in a display case. Moving is an impossibility even as Gaster takes the few steps closer, as he boxes Sans into the wall, grabbing him by his chin. 

The kiss that follows is as surprising as it is unwelcome, Gaster’s tongue forcing itself into the cavity of Sans’ mouth, licking at the back of his teeth. The taste of other’s magic is unpleasant, bland and metallic, like cold liquid steel filling his mouth. His magic responds to the intrusion, forming a tongue to push away the invasive appendage, but to no avail. 

The other monster finally draws back, his tongue snaking back into the cavity of his mouth. Sans wants to spit the remnants of the foreign magic out, wants to turn away - but Gaster’s hand forces him to look straight ahead, to look at Gaster. 

“Know what you have to bargain with,” Gaster tells him, licking at his own mouth as if there was an aftertaste of Sans’ own magic on it. “It doesn’t matter if you let me, Sans.” 

He lets Sans drop to the ground. Sans staggers against the wall, knees buckling at the impact; Gaster’s hands materialize to hold him up. 

“I already had you once,” the skeleton continues, his voice soft and low, almost intimate. Almost like a lover, yet nothing like one. “And I know I can always have you again.“ 

_oh_ , Sans thinks detachedly, staring up at the scientist in the way a monster would look at their own recently detached limb and think, _that’s my hand_. He was wrong. 

He never had any leverage in the first place. 

* * *

The door shuts behind Papyrus. 

A lone white hand rises up from behind the controls. It snaps its fingers once, and dissolves. 

* * *

In a burst of clarity, Sans recalls: 

A challenger for the throne, defeated and broken. He lies, sprawled across the thorns of the throne room, Asgore and his mighty trident before him. The crowd roars, baying for the dust of the loser to be the nutrients of the garden, to be the testament of Asgore’s insurmountable strength. 

Instead, the monster had gotten up, clutching his chest. Sans stops himself from picking the next pocket he had found left unattended, watching as the monster shudders, as strings of woven light spins into existence in the palm of his hand. It forms the monster’s soul, delicate and even beautiful in this hellhole that they all lived in, and for a fleeting moment, Sans is entranced. 

“I offer my soul as your slave,” the monster croaks, and extends his soul towards the king. 

Asgore stares down at the monster. The crowd quiets, waiting for the verdict. 

“No,” he finally says, and Sans turns away as he crushes the soul within his paws. 

* * *

“wait,” Sans utters. 

Gaster pauses, staring at him, sockets widening in blank surprise. Sans takes the chance to speak now - if he delays, he will never be able to say it. He will be stuck in this complex, forever wondering what happened to his little brother, forever regretting his life. 

And if this fails, well. Death would be a blessing more than a curse. 

He raises his hand, willing his soul out into the open. He’s never done so before, remembering how easy it was for Asgore to destroy, and had never dared to even in front of Papyrus - but if anything, it seems to be instinctual. Perhaps his magic understands his desperation, his need to give something, to have something he can save Papyrus with even at the expense of his own life - and that is why the faintest of threads shimmer in his hand, weaving into itself, forming what he comes to know as his soul. 

His eyes dart to Gaster’s face. The look on the other monster’s face is best described as intrigued. 

(He has a chance yet.) 

The soul pulses, gentle warmth in his hand, and his instincts tell him to hold it close, to keep it from harm. It’s only the thought of Papyrus that makes his arm stretch out, an offering. 

A sacrifice. 

“let me go after my brother,” Sans says. He doesn’t say ‘please’ - his lesson is learnt. The words of that monster, now dust among dust spread over the flowers of Asgore’s garden, echoes in his head; his voice does not waver. “i offer my soul as yours." 

Gaster is quiet. The dim light of Sans’ soul casting shadows around the both of them, illuminating their own world. In any other universe, perhaps it would have been romantic. Perhaps sharing one’s soul would have been an expression of trust, and not one of weakness. 

And perhaps it’s curiosity that comes over Gaster, instead of his rationality. 

“Do you know what you are offering?” The scientist murmurs, almost in a daze. His hands come up, almost as if he is about to touch, only to refrain from doing so. “Why… you would do so much for your brother?" 

Why was, is never a question Sans had to ask. 

“because i love him,” he says. 

Gaster laughs, soft and unbelieving. There’s a part of Sans that thinks perhaps he has never known love, and there’s a part of Sans that despite everything, pities him. 

“And that will be your downfall,” Gaster finally says. His hands wrap around Sans’ soul, more delicately than he thought the other monster would bother to, and pulls it towards himself. Sans feels the ghost of a tug as Gaster brings it closer to himself, feels the alarm thrumming low in his marrow. Gaster might kill him, might crush his soul in his hands. 

The only saving grace is that if he does, maybe Sans won’t feel a thing. 

“I accept,” Gaster says. 

Several things happen at once. 

Heat rushes through his entire body, leaving his bones chilled in its wake. He feels himself toppling backwards towards the wall, only to be caught by Gaster’s hand around his shoulder. 

In the palm of Gaster’s hand, his soul begins to melt into something formless. Wisps of purple magic float towards the mass of white, swirling into and tinting the soul a pale lilac. In the back of his skull, something wordless echoes - flashes of thoughts, images, _emotions_. 

Determination, Sans realises. His or Gaster’s, he doesn’t know. 

Gaster tilts his head back, and brings the hand holding Sans’ soul to his mouth. The soul shimmers, now clearly liquid, held intact by Gaster’s magic. 

“Drink,” Gaster says. 

Sans drinks. 

It tastes like nothing. The liquid slips down his throat, merging with his magic with a sense of relief, a sense of coming home - but there’s something else. Something cold, something aloof. 

The echoes solidify, and Sans _feels_. 

The emotions pouring into him feel more like a tidal wave - a confusing jumble of intentions. The strongest is curiosity in its most detached form, a need to see what happens next, what the results might be; the rest is a mixture of dark satisfaction, of disbelief, of expectations - and an underlying current of the need to be in control. 

And now he has control. 

Gaster takes his hand away. He looks down at Sans, and the curiosity returns in full force, with a hint of lust. 

“Kiss me,” he orders, the words echoing in Sans’ head along with a whisper of _wantwantwant_ \- and Sans is reaching up and putting his mouth to Gaster’s before he even realizes it. 

Pleasure curls through him - pleasure he recognizes as Gaster’s. Sans jerks back, his back hitting the wall and his hand flying up to his mouth. But Gaster only smiles at him, the grin starting small and growing wide, fed by the feeling of power, of being in control. 

“You can go find your brother now,” his voice all but a purr. 

Sans stares up at the scientist, before turning and walking away. His walk turns into a trot, before turning into a run - the slap of his shoes against the ground a constant beat of self-doubt. 

_what have you done? what have you done whathaveyoudone -_

**_You will come back_** , Gaster says in his head, and Sans runs.

**Notes for the Chapter:**

> u fucked up sans

**Author's Note:**

> [ pssst. too little chapters? ](http://www.sweetsinnerchild.tumblr.com/)

**Works inspired by this one:**

  * [Take All (Leave Nothing)](https://archiveofourown.org/works/6857218) by [LadyHorizon94](https://archiveofourown.org/users/LadyHorizon94/pseuds/LadyHorizon94)




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